1. Field of the Invention
The present invention relates to non-isolated power supplies for supplying a load with a low D.C. voltage obtained from a high A.C. voltage. Such A.C./D.C. converters are found in most electric household appliances to supply low-voltage components such as, for example, logic circuits (microprocessors, programmable logic), electromechanical actuators (relay) or electronic actuators (triacs).
2. Discussion of the Related Art
In such applications intended for being supplied by the mains, the converters must respect electromagnetic compatibility standards (in particular, standards EN 55022, EN 55014, IEC 1000-4-11, IEC 1000-4-5).
Two solutions capable of fulfilling these standards while providing a sufficient power (on the order of one watt) for the supply of most electronic cards are known.
FIG. 1 very schematically illustrates a first conventional A.C./D.C. converter. Such a converter is based on the use of a so-called "class X" high voltage capacitor C connected, in series with a zener diode DZ, between two terminals E1, E2 that receive a high A.C. voltage Vac (for example, the 240V/50 Hz or 110V/60 Hz mains). A resistor R of small value (on the order of one hundred ohms) is generally connected between terminal E1 and a first terminal of capacitor C, the other terminal of which is connected to cathode K of diode DZ. The anode of diode DZ is connected to terminal E2. The function of resistor R is to limit the current upon circuit power-on. The junction point K of capacitor C with diode DZ is connected, via a rectifying diode D, to the positive terminal of a low voltage capacitor C' across which is sampled a D.C. output voltage Vs that supplies a load Q between respectively positive and reference output terminals S1 and S2. In the present case of a non-isolated power supply, terminal S2 is confounded with terminal E2 to which is connected the negative terminal of capacitor C'.
High voltage capacitor C behaves as a reactive inductance enabling limiting the current in the load. During positive halfwaves of voltage Vac, capacitor C lets a current run through diode D and, accordingly, through filtering capacitor C' and into the load. As soon as the amplitude of voltage Vac becomes greater than the threshold voltage of diode DZ (neglecting the voltage drop in resistor R), the current of capacitor C is shunted by diode DZ, which thus enables regulating output voltage Vs to the value of this threshold voltage. During the negative halfwaves of voltage Vac, diode D is reverse biased and the load is supplied by the power stored in capacitor C'.
It should be noted that the example of FIG. 1, described in relation with an A.C. voltage Vac and a halfwave rectification by means of diode D, may also be implemented by placing a rectifying bridge between terminals K and E2. Filtering capacitor C' is used to make voltage Vs across load Q substantially constant.
FIG. 2 illustrates, still very schematically, a second conventional example of a converter from an A.C. high voltage to a D.C. low voltage.
As previously, a high A.C. voltage Vac is applied between two input terminals E1, E2 of the converter. In the example of FIG. 2, the converter is formed from a low frequency transformer T, that is, a transformer operating at the frequency of the system supplying A.C voltage Vac. The two terminals of a primary winding L1 of transformer T are connected to terminals E1 and E2. The two terminals of a secondary winding L2 of transformer T provide a low A.C. voltage Vi, the peak amplitude of which corresponds, in this example, to the value of D.C. voltage Vs desired across output terminals S1, S2 of the converter to supply load Q. As in the example of FIG. 1, voltage Vs is sampled across a low voltage filtering capacitor C'.
A.C. low voltage Vi is applied to two A.C. input terminals 2, 3, of a diode bridge D1, D2, D3, D4, the rectified output terminals 4, 5, of which are connected to terminals S1 and S2. As previously, this is a non-isolated power supply, terminals E2 and S2 forming a single terminal.
In the example of FIG. 2, the value of output voltage Vs is determined by the transformation ratio, that is, by the ratio between the number of spirals of windings L1 and L2 of transformer T.
The above-described solutions are relatively simple to implement due to the small number of components that they use. Further, the components can be sized so that the converters fulfill the requirements of electromagnetic interference standards. Moreover, these solutions allow direct control of bidirectional switches (for example, triacs) since they have a common point with the supply system.
However, the two conventional solutions described here-above have the essential drawback of being bulky and expensive.
The high cost of the first solution is linked to the use of a class X capacitor having to withstand the voltage of the supply system. Such a capacitor is also characterized by a large bulk. For these reasons, converters implementing this solution are generally limited to small currents (under 30 mA).
For the second solution, the high cost and the large volume result from the use of a transformer. Further, this second solution has the disadvantage of generating significant losses due to the use of the transformer. This second solution is more specifically intended for converters meant to provide larger currents (between 30 and 80 mA).